The present invention relates to a system and method for controlling ignition timing for an internal combustion engine especially but not exclusively installed in an automotive vehicle, in which a fail safe function is provided against malfunction of an engine knock detection circuitry in the ignition timing controlling system.
Ignition timing needs to be determined according to the operating state of internal combustion engines so that the engine may be operated at optimum efficiency.
Therefore, taking efficiency and fuel consumption of the engine into account, it is most preferable to ignite the fuel-air mixture supplied to each engine cylinder in the vicinity of a minimum advance angle when it will produce the maximum torque output, the so called MBT (Minimum advance for Best Torque). Therefore, the ignition timing controlling system functions to detect the operating condition of the engine and derive therefrom the optimum spark advance timing and to cause spark plugs within engine cylinders to be activated at the optimum timing.
However, under certain engine operating conditions, such as high engine speed or heavy engine load stable engine operation cannot be achieved due to the occurrence of engine knocking when the ignition timing is advanced to the MBT. In addition, such atomospheric conditions as temperature and humidity may affect the operating range in which knock may occur.
To cope with the problems described above, knock control systems have been developed in which the absence or presence of engine knock is detected and the ignition timing is controlled accordingly. There is, however, a problem in such knock control systems as described in that should the knock detection circuitry fail, to produce the knock detection information suitable for ignition timing control, i.e. retard of ignition timing advance angle cannot be carried out. Consequently, heavy vibration may occur in the engine due to the occurrence of knocking and in a worst case the engine may be destroyed.
To cope with the problem described above, it is desirable to adjust the ignition timing to an ignition timing angle at which engine knocking does not occur when the knock detection circuitry produces an alarm indicative of knocking.
A Japanese Patent Application First Publication (Tokkai) No. Sho 60-20584 published on May 22, 1985 exemplifies a conventional ignition timing controlling system for an internal combustion engine in which a fail safe function is provided against malfunction of the knock detection circuitry.
In the conventional ignition timing controlling system disclosed in the above-identified Japanese Patent document, the system is capable of detecting malfunction in the knock detection circuitry and provides a fail safe function by retarding the ignition timing angle when the malfunction in the knock detection circuitry is detected in order to avoid damage to the engine due to knocking. On the other hand, when load or engine speed is low and the likelihood of engine knocking is extremely low (hereinafter, this condition shall be referred to as a particular operating range), the above-described fail safe function is released (disabled) in the system disclosed in the above-identified Patent document to prevent the reduction of engine output performance accompanied with an inappropriate spark advance setting which may occur due to detection by the system of conditions that simulate, but are in actuality not, engine knock.
However, since the conventional ignition timing controlling system disclosed in the above-identified Patent document releases the fail safe function which would be performed in the particular operating range described above to avoid the reduction of engine performance that would accompany erroneous detection of knock. The fail safe function is not performed by the knock detection system if engine knocking occurs in the particular operating range. Although the occurrence of knocking in the particular range is quite infrequent it is still unacceptable in that it reduces engine performance and can cause damage or wear to the engine.